1. Field of the Invention
The present invention relates to a pressure control valve that can variably control a set pressure of relief valves by pilot signal pressure supplied from an outside.
More particularly, the present invention relates to a pressure control valve that can variably control the set pressure of a relief valve in accordance with pilot signal pressure controlled in an operator's seat and so on when the set pressure of the relief valve required in a working device (e.g. an option device) is changed.
2. Description of the Prior Art
In general, a pressure control valve is used within a set pressure of a hydraulic circuit. In the case of replacing a working device in accordance with working conditions, the set pressure of the relief valve is reset in accordance with the replaced working device.
In the case of controlling the set pressure of the relief valve, it is required for an operator to set the pressure of the relief valve as an operator directly confirms a pressure gauge, and this causes the operator inconvenience. In consideration of such inconvenience in use, a device, which enables an operator to control the set pressure of the relief valve by controlling pilot signal pressure on the outside of a cab or in the operator's seat, has been used.
As illustrated in FIG. 1, a conventional pressure control valve includes a first plug 1 having an inlet port 1a into which high-pressure hydraulic fluid from a hydraulic pump P flows and a tank path 1b communicating with a hydraulic tank T; a poppet 2 slidably coupled within the first plug 1 to open/close a flow path between the inlet port 1a and the tank path 1b; a poppet spring 21 elastically supporting the path between the inlet port 1a and the tank path 1b in a closed state by pressing the poppet 2; a guide 20 supporting the poppet spring 21; a pilot poppet 19 draining the hydraulic fluid in a pressure chamber c of the first plug 1 to the hydraulic tank T by opening a flow path of a valve sheet 3 when high pressure that exceeds a preset pressure is generated in the pressure chamber c; a pilot poppet spring 18 elastically supporting the path of the valve sheet 3 in a closed state by pressing the pilot poppet 19; a piston 17 supporting one end of the pilot poppet spring 18, and slidably moving within a screw plug 11 to control the set pressure of a relief valve when pilot signal pressure is supplied; a spring elastically supporting the piston 17; a sleeve on which a pilot port 7a for supplying the pilot signal pressure to the piston 17 is formed; a fastening nut 10 and a second plug 12 fixed to an outer surface and an inner surface of the screw plug 11, respectively, to prevent a pressure change after the set pressure of the relief valve is set.
Hereinafter, the operation of the conventional pressure control valve will be described with reference to the accompanying drawings.
A) In the case where the set pressure is used (i.e. the pilot signal pressure is not supplied from an outside):
Hydraulic fluid discharged from the hydraulic pump P flows into the pressure chamber c of the first plug through the inlet port 1a and an orifice in the poppet 2 in order. At this time, due to a difference in sectional area between hydraulic pressure parts of the poppet 1, the poppet 2 is pressed in right direction as shown in the drawing to keep the flow path between the inlet port 1a and the tank path 1b in a closed state.
If the pressure of the hydraulic fluid supplied to the pressure chamber c reaches the set pressure of the pilot poppet spring 18, the pilot poppet 19 moves in left direction as shown in the drawing to open the flow path of the valve sheet 3. Accordingly, the hydraulic fluid in the pressure chamber c is returned to the hydraulic tank T through a path 5.
Simultaneously, due to the opening of the pilot poppet 19, the pressure of the hydraulic fluid in the pressure chamber c is instantaneously lowered. Accordingly, the poppet 2 moves in left direction as shown in the drawing, and the hydraulic fluid is supplied from the inlet port la to the hydraulic tank T through the tank path 1b. At this time, the set pressure is determined in accordance with the elastic force of the pilot poppet spring 18 set by the control of the screw plug 11.
B) In the case where the set pressure is changed (i.e. the pilot signal pressure is supplied from an outside):
The pilot signal pressure from an outside is supplied to the piston 17 through a pilot port 7a of the sleeve 7 and a path 11a formed in the screw plug 11, and thus the piston 17 moves in upper left direction as shown in the drawing (in this case, the spring 14 is compressed).
Accordingly, since the elastic force of the pilot poppet spring 18 is reduced, the set pressure value generated in the hydraulic pump P is reduced, and the set pressure of the relief valve is varied.
In the case where a pipe (not illustrated) for supplying the pilot signal pressure from an outside is jointly connected to a single pilot port 7a, it is interfered with components around the relief valve, and thus it is impossible to jointly connect the pipe to the pilot port 7a. 
Also, in the case where a drain hole 15 formed in the screw plug 11 is exposed to an outside and a foreign substance or rainwater flows in through the drain hole 15 after an external coating work, the hydraulic driving surface of the spring 14 or the piston 17 is corroded to shorten the life span of the corresponding component.
Also, frictional resistance is generated by O-rings 9 and 16 mounted to seal a gap between mutual contact surfaces of the piston 17 and the screw plug 11, and an O-ring 13 mounted to seal a gap between mutual contact surfaces of the piston 17 and the second plug 12.
Accordingly, as illustrated in FIG. 2, a hysteresis occurs, whereby the set pressure of the relief valve against the pilot signal pressure supplied from an outside cannot be accurately controlled.
Also, the hydraulic fluid fed from the hydraulic pump P and waiting in the pressure chamber c may leak to the hydraulic tank side T due to a difference between the inner diameter of the first plug 1 and the outer diameter of the poppet 2.
At this time, if load is generated on the inlet port side 1a of the first plug 1, a safety accident, such as falling of a working device (e.g. boom or the like), may occur.
In addition, in the case of setting the minimum pressure of the relief valve, the range of the minimum pressure is enlarged due to tolerance of components constituting the relief valve, and thus it becomes impossible to form the pressure desired by the user (e.g., in the case where the minimum pressure of the relief valve is 100 kg/cm2, the actual pressure may be set to 110 kg/cm2.